Replace a few uses of SelectionDAG::getTargetNode with

Replace a few uses of SelectionDAG::getTargetNode with
SelectionDAG::SelectNodeTo in the instruction selector. This
updates existing nodes in place instead of creating new ones.

Go back to selecting ISD::DBG_LABEL nodes into
TargetInstrInfo::DBG_LABEL nodes instead of leaving them
unselected, now that SelectNodeTo allows us to update them
in place.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53057 91177308-0d34-0410-b5e6-96231b3b80d8